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«E & ' h Y z D Tag des Kolloquiums: 18. Juli 2011 Gutachter: 1. Prof. Dr. Burghard W. Flemming 2. Prof. Dr. Katrin Huhn Prüfer: 1. Prof. Dr. Dierk ...»

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E

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h

Y z

D

Tag des Kolloquiums:

18. Juli 2011

Gutachter:

1. Prof. Dr. Burghard W. Flemming

2. Prof. Dr. Katrin Huhn

Prüfer:

1. Prof. Dr. Dierk Hebbeln

2. Prof. Dr. Tobias Mörz Acknowledgements I would like to deeply thank my advisor, Prof. Dr. Burghard W. Flemming, for his continuous support, guidance and encouragement on my research presented in this dissertation. He has provided me with the freedom and means to explore the complex but charming coastal and estuarine systems. His constant enthusiasm inspired me to make greater efforts. He also told me “carry on regardless” to encourage me to pursue my dream.

I wish to thank all the staff of the Senckenberg Institute and the administrative group of the Bremen International Graduate School for Marine Sciences (GLOMAR) for their cooperation and assistance. Special acknowledgements for their help and discussions are due to Prof. Dr. Dierk Hebbeln, Dr. Alexander Bartholomä, Dr.

Christian Winter, Dr. Verner B. Ernstsen, Dr. Adam Kubicki, Dr. Chang Soo Son, and Dr. Li Wang. Thanks are due to the Coastal Laboratory at Nanjing University, including Prof. Dr. Shu Gao, Prof. Dr. Yaping Wang, Dr. Yang Yang, Yunwei Wang.

As I would not have been able to carry out my research without financial support, I am grateful to the Chinese Scholarship Committee (CSC) for providing a generous scholarship. Senckenberg Institute is acknowledged for providing the working space and facilities, and occasional travelling funds. GLOMAR is acknowledged for organizing lectures, seminars and providing travelling funds to attend international conferences. I would like to express my sincerest gratitude for this support.

I II Summary The tide acts on coastal and estuarine systems, amongst others by inducing (1) sediment transport and, as a consequence, (2) morphological change.

In tidal environments the response of suspended sediment concentration (SSC) to the current velocity is not instantaneous, the SSC lagging behind the velocity (phase lag), and the amplitude of SSC variation decreasing with height above the bed (amplitude attenuation). In order to quantitatively describe this phenomenon, a one-dimensional (1D) vertical advection–diffusion equation of SSC is derived analytically for uniform unsteady tidal flow by defining a concentration boundary condition using a constant vertical eddy diffusivity and sediment settling velocity. The solution, in simple and straightforward terms, shows that the vertical phase lag increases linearly with the height above the bed, while the amplitude of the SSC variation decreases exponentially with the height. The lag of sediment movement or “diffusion/settling lag” is the mechanism generating the phase lag effect.

In coastal and estuarine waters, depth-averaged suspended sediment concentration (DASSC) at a fixed observation site has two sources: local resuspension and advection along a horizontal gradient. The empirical decomposition method can separate the horizontal residual suspended sediment flux (HRF) into several terms, e.g., Eulerian flux, Stokes’ drift, and tidal pumping. A simple depth-averaged 1D model is solved analytically for an observational site to explore the determinants of the forcing factors comprising residual, M2 and M4 current velocities, mean and M2 water depths, and the DASSC gradient relative to the tidal variations of DASSC and HRF. Through the solutions, the relative contributions of local resuspension and advection are clarified, the empirical decomposition terms having clear physical explanations. The solutions are applied to fit and explain the observations at a fixed station in a macro-tidal channel located along the Jiangsu Coast, China.

Tidal basins can be characterized by two major morphological elements: tidal

III channels and tidal flats. Some scale-dependent empirical relations have been suggested based on observations in tidal basins of the Dutch and German coast. Thus, the channel area and volume are proportional to the 1.5 power of the basin area and tidal prism, respectively, whereas the relative channel area and the ratio between channel volume and tidal prism are both proportional to the square root of basin area.

The coefficients before the power in these relations are of the order of 10-5. In order to provide physical explanations for these relations, a theoretical model and a two-dimensional horizontal numerical model are developed. They result in the same type of scaling relationships and the same order of coefficients, and the morphological response of changing grain size and tidal amplitude can be successfully predicted.

Hypsometry is the distribution of volume or horizontal surface area with respect to elevation. Some observations show contradictory scale-dependent characteristics of tidal flat hypsometries in back-barrier tidal basin environments, and traditional theory explains the flattening hypsometry by relating concave-up hypsometries in low tidal range basins to the dominating influence of wind waves rather than tidal currents. In order to investigate these two problems, two series of two-dimensional depth-averaged (2DH) numerical modelling exercises were carried out. The results show that, large basin areas and low tidal ranges favour strong concave-up hypsometries, whereas small basin areas and high tidal ranges favour less concave-up hypsometries. In addition to the traditional theory, the flattening of hypsometric profile shapes can therefore also be interpreted as a response to the relative area of intertidal areas or channels in the tidal basins, and that strong concave-up hypsometries are formed which are associated with relatively large areas of low tidal flats and small areas of high tidal flats. As basins with large areas and low tidal ranges have large relative channel areas, this results in pronounced concave-up hypsometries, and vice versa.





The Qiangtangjiang Estuary (the outer part being known as Hangzhou Bay) located on the east coast of China is a large funnel-shaped, tide-dominated and well-mixed estuary. The estuarine morphology is characterized by a large sand bar having a total IV length of 125 km and an elevation of 10 m above the average depth of the adjacent seabed. In order to investigate the physical processes governing the formation of this morphological feature, 2DH process-based morphodynamic modeling was carried out.

The model simulated a 6,000-year period, the output showing the development of a sand bar that reached equilibrium within about 3,000 years. The general shape, size and position of the modeled sand bar are consistent with the observations. A series of sensitivity analyses suggest that the estuarine convergence rate, sediment supply, and river discharge are the main controlling factors of sand bar formation. Similar to other large funnel-shaped, tide-dominated estuaries of the world, a sufficient supply of fine cohesionless sediment (derived from the adjacent Changjiang Estuary), a large river discharge, and a strong shoreline convergence rate have shaped the large sand bar in the Qiangtangjiang Estuary.

VVI Contents

Acknowledgements

Summary

1. General Introduction

1.1. Rationale and scope

1.1.1. Tide-induced suspended sediment transport

1.1.2. Tide-induced equilibrium morphology

1.2. Methods

1.2.1. Empirical modeling

1.2.2. Analytical modeling

1.2.3. Numerical modeling

1.3. Validation

1.4. Individual studies

2. Tide-induced Vertical Suspended Sediment Concentration Profiles:

Phase Lag and Amplitude Attenuation

Abstract

2.1. Introduction

2.2. Analytical solution

2.3. Validation

2.4. Discussion

2.5. Conclusions

3. Tide-induced Suspended Sediment Transport:

Depth-averaged Concentrations and Horizontal Residual Fluxes Abstract

3.1. Introduction

3.2. Analytical solution

3.3. Observations and Applications

3.4. Discussion

VII 3.4.1. Influence of advection

3.4.2. Influence of the deposition coefficient

3.4.3. Applicable scope of the solution

3.5. Conclusions

Appendix 3A. Interpretation of eight harmonic terms in the solution of DASSC..................55 Appendix 3B. Interpretation of the terms F1 to F5 in the solution of HRF

4. Scale-dependent Characteristics of Equilibrium Morphology of Tidal Basins along the Dutch-German North Sea Coast Abstract

4.1. Introduction

4.2. Study area

4.3. Empirical model

4.4. Theoretical model

4.5. Numerical model

4.5.1. Methods and Setting

4.5.2. Results

4.5.3. Sensitivity analysis

4.6. Discussion

4.6.1. Comparison of empirical, theoretical and numerical modeling results.................78 4.6.2. The mechanisms of the formation of scale-dependent morphology

4.6.3. Applications to morphological changes induced by reclamation and sea level rise

4.7. Conclusions

5. Modelling the Equilibrium Hypsometry of Back-barrier Tidal Flats in the German Wadden Sea (southern North Sea) Abstract

5.1. Introduction

5.2. Study area

5.3. Methods

5.3.1. Model description

5.3.2. Model setting

5.3.3. Data processing

5.4. Results

5.4.1. Equilibrium state examinations

VIII 5.4.

2. Scale effects

5.4.3. Tidal range effect

5.5. Discussion

5.5.1. Comparison with observations

5.5.2. Basin scale and tidal range control on the tidal flat hypsometry

5.6. Conclusions

6. Modeling the Formation of a Sand Bar within a Large Funnel-shaped, Tide-dominated Estuary: Qiantangjiang Estuary, China Abstract

6.1. Introduction

6.2. Study area

6.3. Methods

6.3.1. Model description

6.3.2. Model setting

6.3.3. Sensitivity analysis

6.4. Results

6.4.1. Reference case R

6.4.2. Sensitivity analysis

6.5. Discussion

6.5.1. Comparison with the Qiangtangjiang Estuary

6.5.2. Comparison with other numerical modeling results and other large funnel-shaped tide-dominated estuaries

6.5.3. The unique position of the Qiangtangjiang Estuary within the general framework of estuarine morphology

6.6. Conclusions

7. Concluding Remarks and Perspectives

7.1. Tide-induced suspended sediment transport

7.2. Tide-induced equilibrium morphology

References

–  –  –

1. General Introduction

1.1. Rationale and scope The tide rises, the tide falls. This cyclical force acts on coastal and estuarine systems, amongst others inducing (1) sediment transport and, as a consequence, (2) morphological change. This constant change has aroused the curiosity of man for thousands of years (e.g. Pliny the Elder, ca. 45 AD; Dou, ca. 762-779 AD). In modern times, this former curiosity has given way to purposeful investigation as an existential need because some 60% of humankind today lives within 50 km of the coast (Pernetta, 1994). This is putting enormous pressure on coastal systems and their natural resources and, in the context of climate change and the predicted acceleration in sea-level rise (Parry et al., 2007), the associated environmental problems are expected to increase with time. In addition, new and perhaps unforeseen problems will emerge that will require responsible action if the effects of natural hazards are to be mitigated and social disasters avoided. Responsible action, however, requires a sound understanding of the issues at hand. In a geological context, this means a thorough understanding of the physical processes governing sediment transport and resulting morphological change. The present dissertation attempts to contribute towards such understanding, in particular by the application of modern modeling techniques to unravel some hitherto unresolved issues.



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